Efficient and cost effective spray cooling of electronic assemblies requires high-density system packaging. Two problems related to high-density system packaging include heat removal and electromagnetic interference (EMI) shielding.
High density system packaging results in heat removal problems caused by liquid and vapor leaving the surfaces of integrated circuits (chips) interfering with and disrupting the sprays which are directed at the surfaces of the same and other chips. Liquid and vapor leaving one chip can cause poor heat-transfer performance in adjacent chips. This is particularly likely to occur in the very common circumstance where a high-powered chip, requiring a large volume flux of spray, is adjacent to one or more lower-powered chips requiring less spray volume. Fluid coating the high-powered chip's surface absorbs heat and leaves as a mixture of liquid and gaseous vapor. The volume and velocity of this fluid movement can be considerable, and due to lack of control over its direction and movement, it is particularly likely that it will interfere with the spray directed at adjacent chips. This interference can cause flooding of the device which inhibits thin film evaporation, thereby reducing the heat transfer performance. This can result in over-heating of these adjacent chips.
One possible solution is to increase the spray volume directed at lower-power chips. This solution is flawed for several reasons. First, excess coolant carried on the surface of the lower-powered chips can actually reduce the quantity of coolant which undergoes a heat-absorbing phase-change. Where more than a thin film is used to cover a heat generating component the rate of heat transfer is reduced. Secondly, the cumulative effect of an increased spray volume directed by several spray nozzles at several low-powered chips surrounding a high-power chip could result in undesired changes in the spray pattern directed at the high-powered chip. Thirdly, excessive spray requirements necessarily result in a requirement of greater pump capacity, overall greater coolant supply and an associated increase in costs. And finally, since the overall heat produced by the entire system can result in the vaporization of only a fixed quantity of liquid coolant, significantly exceeding that level of coolant supplied will result in a significant excess quantity of liquid coolant. In some applications, the spray module may tend to fill up with liquid. If this coolant is not properly directed, it could swamp chips in its flow pathway, thereby covering such chips with more than the optimal quantity of coolant for maximum efficiency in heat transfer.
A similar possible solution is to employ overlapping sprays to suppress the interference effect. However, in most applications the chips are not spaced in a manner that supports this strategy without unduly increasing the system flow rate requirements resulting in the above problems.
A further possible solution to the problem of interference between the spray directed at adjacent chips is to design the board in a manner in which minimization of this phenomena is a design parameter. However, this significantly increases design costs. More troubling still, it is possible that no arrangement of the chips on the board may result in a satisfactory design in many applications. Also, the design engineer and board layout technician may not be well-versed in thermal issues, and may be unable to achieve satisfactory results.
For the foregoing reasons, there is a need for a fluid control apparatus and method of use that can result in enhanced control over the direction and distribution of spray coolant in a manner that reduces or eliminates the interference caused by spray directed at a first chip with the spray and cooling process of a second chip.
A further problem facing electronic designers is related to EMI shielding. EMI shielding deals with two aspects of electromagnetic interaction, including electromagnetic emissions and electromagnetic susceptibility, i.e. susceptibility to such emissions. Electromagnetic interference (EMI) occurs when the emissions from one component adversely effect another component due to its susceptibility to the emissions.
Heat-generating electronic devices such as multi-chip modules, electronic hybrid assemblies such as power amplifiers and others passive system components, such as transformers or inductors, operating at high frequencies are frequently sources of electromagnetic emissions. An even wider number of active and passive components, such as preselect filters and receiver front end low noise amplifiers, are adversely effected by the EMI, and are therefore considered to be electromagnetically susceptible. EMI must therefore be attenuated to keep the system functioning properly. Similarly, governmental and other worldwide regulatory authorities have set forth requirements for EMI emissions that must be taken into consideration.
Electromagnetic compatibility may be attained where EMI emissions are attenuated or shielded by grounded enclosures and other similar shields which tend to enclose and isolate the EMI producing or EMI susceptible components. Unfortunately, such enclosure is inconsistent with the need to remove heat from the components to improve and maintain reliability. Similarly, isolation of individual components is expensive and is inconsistent with the need to consolidate and concentrate components into ever smaller enclosures.
A further engineering challenge associated with the trend to consolidate components into extremely small enclosures involves waste heat removal. New technologies involving spray cooling have allowed enclosure dimensions to be reduced, while increasing component density and yet maintaining thermal tolerances. Such spray cooling techniques involve use of an atomized spray of dielectric or electrically non-conducting fluid which typically undergoes a phase change from fine liquid droplets to a gas, thereby absorbing heat energy.
However, while such thermal techniques may allow component concentration, increasing component densities may also results in increased electromagnetic interaction which may increase exponentially as component separation distances are reduced. As a result, an apparatus which both shields groups of components for EMI and also provides spray cooling structures is known.
U.S. Pat. No. 5,675,473 discloses a cover made of a EMI-attenuating material, such as metal, which defines a compartment which supports a number of spray nozzles which direct spray at components carried within the enclosure.
Such a strategy does contain the EMI and also allows for heat removal. However, all components carried within the compartment are subject to the EMI produced by all of the components. As a result, the EMI problem is not solved, although EMI may be prevented from leaving the compartment.
Additionally, interaction between the spray emitted from adjacent nozzles may result in difficulties in adequately cooling all of the components.
What is needed is an EMI shielding fluid control apparatus for spray cooling that is adapted to isolate each EMI or heat producing component in a manner that allows for both heat removal and also EMI attenuation. The EMI shielding fluid control apparatus must allow spray coolant to move freely in manner that allows recycling, but must also provide a structure which is adapted to controlling the coolant spray direction, so that interaction between adjacent spray nozzles does not result in inadequate heat removal of any component.